It is known to treat a steel melt by injecting into the molten mass of metal particulate additives which are entrained by a neutral (nonreactive) carrier gas such as argon. As a general rule this is carried out simply by blowing the particulate material through a lance into the melt whose upper surface is covered with slag exposed to the atmosphere at ambient pressure. The temperature at the outlet end of the introduction lance (i.e. the melt temperature) is sufficient to vaporize the particulate materials almost instantly thereby forming vapor bubbles which rise in the melt and react with the impurities or undesirable components of the melt. Due to the high specific gravity of the melt and the low density of the vapor large bubbles are formed whose collective interface between the vaporized additives and the steel melt determines the rate of reaction. When the additive is calcium for desulfurization of the melt, calcium oxide and calcium sulfide inhibit the reaction. In general the total interfacial surface area in low, the rate of reaction small and considerable quantities of the additive reach the surface unreacted.
The method of overcoming this in the past has so far been to inject the material as deeply as possible within the melt. Thus if the material is injected 1 or 2 meters or more below the surface of the melt, the rising bubbles have additional time to react more completely with the material of the melt. However, the injection of the calcium at such depths below the surface of the melt can inhibit its vaporization, since the hydrostatic pressure may exceed the vapor pressure at the melt temperature. Thus the calcium bubbles can form extremely slowly and only attain a size encouraging an efficient chemical reaction as the bubbles approach the upper surface of the melt. Pressurization of the melt, that is enclosing the melt and forming a superatmospheric pressure above it, has proven to be completely ineffective to overcome this disadvantage.